I recently started dating someone I've been friends with for years and have often imagined spending my life with. She's asexual, and I am not and I don't know how to tactfully bring up the idea that I'd be interested in a poly dynamic where I could pursue that stuff with other people but she'd still be my primary. We both know and are okay with friends who are poly but I have no clue how she feels about being involved in a poly dynamic.
The thing about dating someone is that you’ve made a commitment to them. Part of that commitment includes being honest with them, and trusting them to receive important information that you share. Your partner’s asexuality and your sexuality are important parts of your relationship that need to be discussed! If you’re dating, you ought to be able to talk about things like desires and expectations around sex and physical affection.
I wouldn’t open things by asking about polyamory - but definitely have a conversation about your relationship, and what it means for her to be ace dating someone who isn’t, and what she wants and needs from the relationship. Talk about what your hopes are for this new relationship. Be clear. Define your terms - explain what you mean by “dating” and “pursue” and “that stuff” and “primary” and “a poly dynamic.” Give space for both of you to make sure you’re speaking the same language.
The best way to find out how she feels is to ask!